The growing demand for wireless communications has motivated attempts to design radios that permit the integration of multiple circuit components onto a single chip to reduce overall cost, size, and power consumption. For example, a radio receiver and a radio transmitter can be integrated onto a single chip. Recent advances in CMOS semiconductor processing and scaling can facilitate the component integration.
One circuit component that can be integrated with other components on a chip is a voltage-controlled oscillator (VCO) circuit. The VCO can be found in various communication applications, such as in cellular communications. The VCO is one of the building blocks of a phase-locked loop (PLL). In one implementation, a PLL refers to a negative-feedback control system in which the phase of a VCO or phase shifter is brought into alignment, or to a predefined static phase offset, with respect to the phase of a periodic reference signal. In this implementation, the PLL can have a reference port for the input reference signal and a feedback port to couple to the output of the VCO. The PLL can compare the phase of the periodic reference signal and the feedback signal, and vary the phase and frequency of the output of the PLL until the reference and feedback signals are phase and frequency matched.